[RFC/Q] How to create a mail-client plugin? Do we need a registry?

[Neil Martinsen-Burrell]

As has been mentioned before on this list, the best way to control Mail.app on
Mac OS X to use it as a client for `bzr send` is to use Applescript.  It should
be possible to use a separate file containing an Applescript that takes to,
from, text, attachment, etc. as arguments and opens up the message in Mail.app.
 A more pythonic way is to use the python module appscript [1].  This provides
an object oriented python binding to Applescript commands.

In order to avoid introducing an external dependency on Bazaar (at least for Mac
OS X where people set email_client = 'Mail.app') it would be nice to distribute
a plugin that uses appscript to provide a mail_client that works under OS X. 
But, I can't see how it would be possible to do a new mail_client from a plugin.

In particular, cmd_send dispatches based on bzrlib.config.get_mail_client(),
which uses a literal dictionary to map configuration values to particular
MailClient subclasses.  I can't see how to add an additional mapping for a new
key to this dictionary from my plugin without having to monkey-patch
bzrlib.config.  Am I missing something about a way to do this, or is it
currently not possible?

A possible solution is some sort of registry system for mail_clients.  There
seem to be plenty of similar registries in bzrlib (e.g. version-info formats) so
would this be an appropriate use for another one?

-Neil